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United States Patent |
5,650,877
|
Phillips, Jr.
,   et al.
|
July 22, 1997
|
Imaging system for deep ultraviolet lithography
Abstract
A catadioptric reduction system operating in the deep ultraviolet range
projects a reduced image of a mask on a substrate. A reducing optic made
of a material transmissive to deep ultraviolet light has a concave front
face covered by a partially reflective surface and a convex back face
covered by a concave reflective surface surrounding a central aperture.
The partially reflective surface transmits a portion of the light passing
through the mask to the concave reflecting surface, which returns a
portion of the transmitted light to the partially reflective surface. A
portion of the returned light is reflected by the partially reflective
surface on a converging path through said central aperture for producing a
reduced image of the mask on the substrate.
Inventors:
|
Phillips, Jr.; Anthony R. (Fairport, NY);
Michaloski; Paul F. (Rochester, NY)
|
Assignee:
|
Tropel Corporation (Fairport, NY)
|
Appl. No.:
|
514614 |
Filed:
|
August 14, 1995 |
Current U.S. Class: |
359/726; 359/621; 359/626 |
Intern'l Class: |
G02B 017/00 |
Field of Search: |
359/732,731,730,727,726
|
References Cited
U.S. Patent Documents
4953960 | Sep., 1990 | Williamson.
| |
5031976 | Jul., 1991 | Shafer.
| |
5031977 | Jul., 1991 | Gibson | 359/732.
|
5206515 | Apr., 1993 | Elliott et al.
| |
5212593 | May., 1993 | Williamson et al.
| |
5220454 | Jun., 1993 | Ichihara et al.
| |
5241423 | Aug., 1993 | Chiu et al.
| |
5251070 | Oct., 1993 | Hashimoto et al.
| |
5289312 | Feb., 1994 | Hashimoto et al.
| |
5402267 | Mar., 1995 | Furter et al. | 359/732.
|
5461456 | Oct., 1995 | Michaloski.
| |
Foreign Patent Documents |
4203464 | Aug., 1992 | DE.
| |
Other References
"A New Series of Microscope Objectives: I. Catadioptric Newtonian Systems"
by David S. Grey and Paul H. Lee, Journal of the Optical Society of
America, vol. 39, No. 9, Sep. 1949, pp. 719-728.
|
Primary Examiner: Sugarman; Scott J.
Attorney, Agent or Firm: Eugene Stephens & Associates
Claims
We claim:
1. A catadioptric reduction system for deep ultraviolet lithography
comprising:
a reducing optic having a main body made of a transmissive material that
conducts a beam of deep ultraviolet light;
a back face of said reducing optic having a central aperture surrounded by
a concave reflective surface;
a front face of said reducing optic having a partially reflective surface
that transmits a portion of the beam to said concave reflecting surface
and reflects a portion of the remaining beam returned by said concave
reflective surface on a path through said central aperture;
a lens group also made from a transmissive material that conducts the beam
of ultraviolet light for correcting aberrations generated by said reducing
optic; and
a central obscuration blocking a portion of the beam that is not reflected
by said concave reflective surface from passing through said aperture.
2. The reduction system of claim 1 in which said reflective and partially
reflective surfaces of said reducing optic provide substantially all
reducing power of the system for limiting chromatic aberrations
accompanying transmission of a range of wavelengths.
3. The reduction system of claim 2 in which said reflective and partially
reflective surfaces provide a 10-fold reduction.
4. The reduction system of claim 1 in which said reducing optic and said
lens group are aligned with a common optical axis.
5. The reduction system of claim 1 further comprising a substantially plane
parallel plate that is modified to include an aspheric surface to correct
for aberrations generated by said reflective and partially reflective
surfaces.
6. The reduction system of claim 1 in which said concave reflective surface
is modified to include an aspheric surface to correct for aberrations
generated by said reflective and partially reflective surfaces.
7. The reduction system of claim 5 in which said central obscuration is
formed as a stop on said plane parallel plate.
8. The reduction system of claim 7 in which said central obscuration blocks
no more than 15 percent of a diameter of the beam incident upon said
plate.
9. The reduction system of claim 8 in which said central obscuration blocks
no more than 10 percent of the beam diameter.
10. An optical projection system for projecting a reduced image of a first
surface onto a second surface comprising:
a lens group that receives a beam of light passing through the first
surface;
a partially reflective surface for transmitting a portion of the beam;
a concave reflective surface surrounding a central aperture for reflecting
a portion of the transmitted beam;
a central obscuration for blocking another portion of the beam of light
from transmitting between said partially reflective surface and said
central aperture; and
said partially reflective surface being related to said central aperture
and said concave reflective surface for further reflecting a portion of
the reflected beam from said reflective surface on a path through said
central aperture forming a reduced image of the first surface on the
second surface.
11. The projection system of claim 10 in which the first and second
surfaces are parallel to each other and aligned with a common optical
axis.
12. The projection system of claim 11 in which said lens group, said
partially reflective surface, said concave reflective surface, and said
central obscuration are aligned with said common optical axis.
13. The projection system of claim 12 in which said partially reflective
surface covers a front face of a transmissive optic.
14. The projection system of claim 13 in which said front face of the
transmissive optic is a concave surface.
15. The projection system of claim 14 in which said concave reflective
surface surrounds said central aperture on a back face of said
transmissive optic.
16. The projection system of claim 15 in which said back face of the
transmissive optic is a convex surface.
17. The projection system of claim 10 further comprising a substantially
plane parallel plate that is modified to include an aspheric surface to
correct for aberrations generated by said reflective and partially
reflective surfaces.
18. The projection system of claim 17 in which said concave reflective
surface is also modified to include an aspheric surface to correct for
aberrations generated by said reflective and partially reflective
surfaces.
19. The projection system of claim 10 in which said central obscuration is
surrounded by a given diameter aperture of the beam, and said central
obscuration blocks no more than 15 percent of the beam diameter.
20. The projection system of claim 19 in which said central obscuration is
formed as a stop on a plane parallel plate that is modified to include an
aspheric surface to correct for aberrations generated by said reflective
and partially reflective surfaces.
21. The projection system of claim 19 in which said central obscuration
blocks no more than 10 percent of the beam diameter.
22. A method of projecting a reduced image of a mask on a substrate having
a feature size less than 0.2 microns with a beam of deep ultraviolet light
having a wavelength less than 200 NM comprising the steps of:
emitting a beam of light having a band of wavelengths less than 200 NM;
illuminating the mask with the beam;
conditioning the beam with a lens group;
transmitting a portion of the beam through a partially reflective surface;
reflecting a portion of the transmitted beam with a concave reflective
surface surrounding a central aperture; and
further reflecting a portion of the reflected beam with the partially
reflective surface on a path through the central aperture forming a
reduced image of the mask on the substrate having a feature size less than
0.2 microns.
23. The method of claim 22 including the further step of orienting the
reticle and the substrate parallel to each other and aligned with a common
optical axis.
24. The method of claim 23 including the further step of aligning the lens
group, partially reflective surface, and reflective surface with the
common optical axis.
25. The method of claim 22 including the further step of blocking a portion
of the beam that is not reflected by the concave reflective surface from
transmitting between the partially reflective surface and said central
aperture.
26. The method of claim 25 in which the beam has a given aperture diameter
within the lens group and said step of blocking blocks no more than 15
percent of the beam diameter.
27. The method of claim 26 in which said step of blocking blocks no more
than 10 percent of the beam diameter.
28. The method of claim 22 including the further step of adding a plane
parallel plate to the lens group for correcting aberrations generated by
the reflective and partially reflective surfaces.
29. The method of claim 28 including positioning a central obscuration on
the plane parallel plate for blocking a portion of the beam that is not
reflected by the concave reflective surface from transmitting between the
partially reflective surface and said central aperture.
30. The method of claim 22 including the step of forming the concave
reflective surface as an aspheric surface for correcting aberrations
generated by the reflective and partially reflective surfaces.
Description
TECHNICAL FIELD
The invention relates to catadioptric reduction systems for projecting
images with low aberrations and to exposure systems for microlithographic
manufacture with deep ultraviolet light.
BACKGROUND
Microelectronics, including semiconductors, storage devices, and flat panel
displays, are generally fabricated in successive layers using
photolithographic techniques for patterning surface features. A reticle or
mask having a predetermined pattern is evenly illuminated and projected
onto a layer of photoresist on the surface of the microelectronic
substrate. Exposed portions of the photoresist are chemically altered,
rendering them more or less soluble to a developer that removes the
soluble portions leaving a positive or negative image of the mask.
High resolution of the surface features is, of course, important; and
improved resolution is continually sought for making the surface features
smaller and more closely spaced so the resulting electronics can be made
smaller, faster, and cheaper. A resolution dimension "R" representing
minimum feature size is related to light wavelength ".lambda.", numerical
aperture "NA", and a process related constant "K.sub.1 " as follows:
##EQU1##
Feature size "R" can be reduced by reducing the wavelength ".lambda." or
the process constant "K.sub.1 " or by increasing the numerical aperture
"NA". In production environments, process constants "K.sub.1 " equal to
0.7 to 0.8 are typical, whereas constants "K.sub.1 " as low as 0.5 can be
achieved in laboratory settings. Numerical aperture "NA" and wavelength
".lambda." are also related to depth of focus "Df" as follows:
##EQU2##
A depth of focus "Df" of at least a fraction micron (e.g., 0.5 microns) is
needed to accommodate flatness variations of the microelectronic
substrates and their successive layers. Since numerical aperture "NA" is
raised to a higher power than wavelength ".lambda." in the above
expression for depth of focus "Df", resolution improvements achievable by
enlarging numerical aperture "NA" are much more limited than those
achievable by shortening the wavelength ".lambda.".
Wavelengths less than 300 nanometers (NM) can be practically transmitted by
only a few optical materials such as fused (synthetic) quartz and fluorite
(calcium fluoride). The transmissivity of even these materials
deteriorates at wavelengths in the deep ultraviolet range less than 200 NM
so a minimum number of optical elements is desirable.
Although it is advantageous to minimize feature size of the images
projected onto the microelectronic substrates, the feature size of the
masks should remain large enough to manufacture efficiently and to avoid
errors from mild levels of contamination. For example, it is important
that small specks of contamination do not bridge features of the masks.
Mask size can be maintained by optically reducing the projected image of
the mask with respect to the mask itself.
Laser light sources operating within the ultraviolet and deep ultraviolet
ranges emit light within narrow bands of wavelengths. However, even narrow
bands of wavelength cause significant chromatic aberrations in
single-material lenses with finite focal lengths. On the other hand,
limiting laser output to a single wavelength is inefficient. Accordingly,
catadioptric imaging systems have evolved which use reflective optics
(mirrors) to reduce image size in combination with refractive optics
(lenses) to compensate for symmetrical aberrations of the reflective
optics.
Beamsplitters or partially reflective mirrors are used to separate light
traveling to and from the reflective optics. Beamsplitters and partially
reflective mirrors, particularly when subjected to angularly diverging
beams, introduce additional aberrations requiring correction. The
beamsplitters also add to the complexity of the imaging systems by
misaligning the object and image planes.
A typical catadioptric optical reduction system used for microlithographic
projections is disclosed in U.S. Pat. No. 5,241,423 to Chiu et al. A
concave spherical mirror provides a four to five times reduction in the
projected image size with respect to a mask, and a beam-splitting cube
separates light beams traveling to and from the mirror. Groups of
refractive optical elements located on opposite sides of the
beam-splitting cube toward both the reticle (mask) and the substrate
correct for aberrations of the mirror and beam-splitting cube.
Chiu et al.'s reduction system is intended for operation at wavelengths of
about 248 NM produced by a KrF excimer laser. However, the large number of
refractive elements and the bulky two prism construction of the
beam-splitting cube limit usefulness of this system at shorter
wavelengths. The transmission of light through fused quartz or fluorite
diminishes with shortening wavelengths, so the number and bulk of
refractive optics must be limited to utilize wavelengths within the deep
ultraviolet spectrum at less than 200 NM length.
U.S. Pat. Nos. 5,251,070 and 5,289,312 to Hashimoto et al. also use a
concave mirror to provide most of the reducing power but use a
semi-transparent mirror on a plane parallel plate instead of a
beam-splitting cube to separate light beams traveling to and from a
concave mirror. The former patent of Hashimoto et al. incorporates plane
parallel refracting plates to correct aberrations caused by the
semi-transparent mirror. The latter patent of Hashimoto et al. uses
high-power refractive optics to collimate the beam transmitted through the
semi-transparent mirror. This reduces aberrations from the
semi-transparent mirror but still requires other refractive optics to
counteract aberrations introduced by the high-power refractive optics.
SUMMARY OF INVENTION
Our invention extends microlithographic manufacture into the deep
ultraviolet spectrum (e.g., less than 200 NM wavelength) for further
reducing the minimum feature size of projected images to less than 0.2
microns. A practical size reticle (mask) is maintained by achieving the
feature size with a highly reduced image of the reticle. The number of
corrective refractive optics is held to a minimum, and the configuration
of optical elements is simplified by maintaining object and image planes
of a reducing system both parallel to each other and aligned with a common
optical axis.
A lens group conditions a beam of light after passing through the reticle.
A reducing optic having specially configured front and back faces projects
a reduced image of the reticle onto a substrate. Both the lens group and
the reducing optic are made from materials that transmit deep ultraviolet
light. The back face of the reducing optic has a central aperture
surrounded by a concave reflective surface. The front face of the reducing
optic has a partially reflective surface that transmits a portion of the
light beam toward the concave reflecting surface and reflects a portion of
the remaining light beam returned by the concave reflective surface on a
converging path through the central aperture. The substrate is aligned
with the aperture for receiving the reduced image of the reticle.
The reflective surfaces of the reducing optic provide the reducing power,
which is preferably a 10-fold reduction in the mask size. The refractive
elements of the lens group and reducing optic exhibit little or no
combined reducing power to avoid chromatic aberrations. Instead, the lens
group corrects at least some of the nonchromatic aberrations generated by
the reducing optic. A substantially plane parallel plate is preferably
incorporated into the lens group and modified to include an aspheric
surface to correct spherical aberrations. The concave reflective surface
of the reducing optic can also be modified to include an aspheric surface
to correct spherical aberrations at an even higher rate.
A central obscuration blocks a portion of the beam of light, which would
not be reflected by the concave reflective surface, from passing through
the central aperture. Preferably, the central obscuration is limited in
size to block no more than 15 percent of projected image. The central
obscuration can be conveniently the beam diameter within the lens group.
More than 15 percent blockage can cause significant degradation in
contrast of the formed as a stop on the plane parallel plate.
DRAWINGS
FIG. 1 is a schematic layout of a microlithographic projection system
arranged according to our invention.
FIG. 2 is an enlarged diagram of our new catadioptric reducing system for
completing the projection of an illuminated mask on a substrate.
DETAILED DESCRIPTION
According to a preferred embodiment of our invention illustrated in the
drawing figures, a laser light source 10 is an Argon-Fluoride excimer
laser that produces a collimated beam 12 of ultraviolet light having a
wavelength bandwidth between 192.6 and 194 NM. A series of three folding
mirrors 14, 16, and 18 convey the collimated beam 12 to an illuminator 20.
Within the illuminator 20, the collimated beam 12 is attenuated and
dispersed by a pair of diffusers 22 and 24 before entering a square
reflecting tunnel 26. The diffuser 22 is adjustable along an optical axis
28 both to control the amount of light entering the reflecting tunnel 26
through the diffuser 24 and to more uniformly disperse the entering light
over an area of the diffuser 24 in common with an entrance 25 of the
reflecting tunnel 26. The amount of separation between the diffuser 22 and
the tunnel entrance 25 controls the amount of excess light that is
scattered beyond the tunnel entrance 25. Together, the two diffusers 22
and 24 produce a wider angle of uniformly dispersed light entering the
reflecting tunnel 26.
The reflecting tunnel 26 functions as a "uniformizer" by dividing the
diffused beam 12 into segments and arranging the segments into a
contiguous array. Unlike most uniformizers, which are made from solid
optical materials such as polyhedral rods or fly's eye lenses, the
reflecting tunnel 26 is hollow with reflective sides to avoid excessive
absorption of the deep ultraviolet light. Such excessive absorption limits
control over the amount of light that can be transmitted through the
illuminator 20 and reduces the useful life of the illuminator 20 by
degrading the optical materials.
A lens group 30 magnifies and projects an image of a plane at an exit 27 of
the reflecting tunnel 26 onto a plane of a reticle 34, which functions as
a mask for microlithographic manufacture of a substrate 36. In addition,
the lens group 30 images a plane at the entrance 25 of the reflecting
tunnel 26 onto a plane at a variable aperture stop 38 within the lens
group 30. The tunnel entrance 25 is imaged at the variable aperture stop
38 as an array of closely knit reflections produced by the reflecting
tunnel 26. The variable aperture stop 38 functions as a mask by excluding
portions of the beam 12 to enhance the diffractive effects of the reticle
34. For example, the aperture stop 38 can take the form of an annular ring
or a series of holes which transmit only selected portions of the beam 12.
The combined effect of the diffusers 22 and 24, which provide a wider
angular dispersion of light entering the reflecting tunnel 26, improves
spatial uniformity of the distribution of light energy throughout the
array of reflections within the aperture stop 38. The dispersion of light
produced by the adjustable diffuser 22 on the diffuser 24 also improves
the spatial uniformity of the distribution of light within each reflection
of the tunnel entrance 25 that comprises the array. The improved spatial
uniformity of the beam 12 at the aperture stop 38 enhances the masking
effect of the aperture stop 38.
Although the uniformizing effects of the diffusers 22 and 24 are most
evident at the aperture stop 38, spatial uniformity is also improved at
the tunnel exit 27, which is imaged at the reticle 34. Thus, the light
beam 12 impinges on the reticle 34 with a uniform spatial distribution of
light energy, while the angular distribution of the impinging light is
controlled by the aperture stop 38 to enhance the contrast of the
reticle's image on the substrate 36.
Our catadioptric reducing system 40, shown in more detail by FIG. 2,
projects a greatly reduced image of the illuminated reticle 34 onto a
surface of the substrate 36. The reticle 34 and the substrate 36 are
oriented parallel to each other and are aligned together with our
catadioptric reducing system 40 along the optical axis 28.
A lens group 42, comprising transmissive optical elements 44, 46, 48, and
50, conditions the beam 12 for entry into reducing optic 52 having a
concave front face 54 and a convex back face 56. The concave front face 54
of the reducing optic 52 is coated to form a partially reflective surface
58 that provides partial transmission uniformly throughout its aperture.
The convex back face 56 is coated in an annular pattern to form a concave
reflective surface 60 surrounding a central aperture 62.
A portion 64 of the beam 12 is transmitted through the partially reflective
surface 58 to the concave reflective surface 60, which returns a
converging beam 66. A portion 68 of the returning beam 66 is reflected by
the partially reflective surface 58 on a converging path through the
central aperture 62 to a point of focus on the substrate 36. The reducing
optic 52 focuses a reduced image of the reticle 34 on the substrate 36.
The lens groups 30 and 42, along with the reducing optic 52, are preferably
made of fused silica for transmitting the beam 12 of deep ultraviolet
light. However, fluorite could also be used. The total refractive power of
the lens group 42 and reducing optic 52 of our catadioptric reducing
system 40 is minimized to avoid chromatic aberrations caused by refracting
the different wavelengths of the output band of the laser light source 10.
The reflective surface 60, along with the partially reflective surface 58,
provides the reducing power; and the lens group 42, along with the
refractive interactions of the reducing optic 52, provides correction for
the systematic aberrations of the reflecting surfaces.
One of the members of the lens group 42 is the substantially plane parallel
plate 48 having a front face 66 and a back face 68. The front face 66 is
planar, but the back face 68 is modified to include an aspheric surface
that corrects for spherical aberrations. The back face 56 of the reducing
optic 52 is also modified to include an aspheric surface to correct
spherical aberrations at an even faster rate.
A central obscuration 70, such as a reflective coating, is applied to the
front face 66 of the plate 48 to block portions of the beam 12 that would
otherwise pass directly through the central aperture 62 without first
reflecting from the reflecting surface 60 of the reducing optic 52. The
plate 48 containing the central obscuration 70 is positioned close to an
aperture stop 72 at which an image of the adjustable aperture 38 is
formed. The central obscuration 70 is relatively small and blocks only
about 10 percent of the diameter of the surrounding aperture, whose outer
diameter is controlled by the aperture stop 72. This converts to only 1
percent of the aperture area. Preferably, the central obscuration is
limited to no more than 15 percent of the aperture diameter or a little
more than 2 percent of the aperture area to minimize undesirable
diffractive effects that reduce contrast of the reticle pattern on the
substrate 36.
Tables 1 and 2 provide prescription information on the preferred
embodiment. All distances are measured in millimeters (mm), curvature is
measured as a radius, but aperture is measured as a diameter. The central
aperture 62 has a diameter of 6 mm. The reduction magnification is 10 fold
and the numerical aperture is 0.6. With a constant "K.sub.1 " assumed at
0.5, the minimum feature size that can be imaged on the substrate is
reduced to 0.16 microns. Features less than 0.20 microns would be possible
under less stringent conditions (e.g., with a constant "K.sub.1 " at 0.6).
TABLE 1
______________________________________
Element
Curvature Aperture
Number
Front Back Front Back Thickness
______________________________________
Object
space 492.0402
44 77.9574 297.6566 61.4083
60.1506
9.7738
space 0.4000
46 49.2496 38.8156 57.8441
52.0798
8.0000
space 22.8357
72 50.5998
space 10.0000
48 plane A(1) 50.4728
50.4321
5.0000
space 16.1580
50 -37.1784 -43.9590 50.3358
55.6548
9.0000
space 0.50000
52 -70.5974 A(2) 56.0455
64.5635
25.7923
space 2.5000
Image
______________________________________
An equation defining the aspheric surfaces "A(1)" and "A(2)" of the plate
48 and reduction optic 52 is given below:
##EQU3##
The coefficient "K" is equal to zero. The coefficient "CURV" and the
coefficients "A" through "D" are given in Table 2.
TABLE 2
__________________________________________________________________________
CURV A B C D
__________________________________________________________________________
A(1)
-1.5899E-04
2.4964E-07
-1.5511E-10
5.5612E-14
-1.8490E-16
A(2)
-1.4052E-02
-6.4218E-10
-8.7217E-13
-8.7864E-16
7.1696E-19
__________________________________________________________________________
Of course, our invention can be practiced with a variety of other
prescriptions operating at other reduction powers and sizes of scale.
Numerical apertures of at least 0.4 are preferred. A single aspherical
corrective surface could be formed on one of the elements including either
the back face 68 of the plate 48 or the back face 56 of the reducing optic
52. Although it is important to limit the amount of optical material
required to transmit the deep ultraviolet light, more corrective elements
could be used with a larger aperture design to provide a larger area of
illumination.
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